Trona deposits in Sweetwater and adjacent counties in Wyoming are found at depths of about 1500 feet underground and consist of a main trona bed varying from 8 to 10 feet in thickness. The trona consist mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3 .2H.sub.2 O) containing about 3 to 15%, or, more generally, 5 to 10% of insoluble impurities, mainly shale.
A typical natural trona composition is given below:
______________________________________ Na.sub.2 CO.sub. 3 41.8% NaHCO.sub.3 33.1% H.sub.2 O 14.1% 89.0% Insolubles: Dolomite MgCO.sub.3 . CaCO.sub.3 5.5% Quartz SiO.sub.2 1.1% Feldspar (K, Na).sub.2 0 . xAl.sub.2 O.sub.3 . ySiO.sub.2 zH.sub.2 O 3.3% Clay 2K.sub.2 O . 3MgO . 8Fe.sub.2 O.sub.3 . 24SiO.sub.2 . 12H.sub.2 O 0.6% Shortite Na.sub.2 CO.sub.3 . 2CaCO.sub.3 0.1% Organic Matter as Elemental Carbon 0.2% Other (by Difference) 0.2% ______________________________________
In the production of sodium carbonate from natural trona, it is usual procedure to calcine the trona at elevated temperature to obtain crude sodium carbonate: EQU 2(Na.sub.2 CO.sub.3.NaHCO.sub.3 .2H.sub.2 O) .fwdarw. 3Na.sub.2 CO.sub.3 + 5H.sub.2 O+ CO.sub.2.
Calcination of the trona effects moisture removal, reduction or elimination of organic matter as well as transformation of the sodium sesquicarbonate to sodium carbonate. However, calcination at elevated temperature also causes sodium carbonate to react with silica contained in the trona to form soluble silicates. The quantity of soluble silicates so formed increases materially with increase in calcination temperature. Soluble silicates are a major impurity in solutions obtained by dissolving calcined trona and, unless steps are taken to reduce soluble silicate content of such solutions, soluble silicates become a significant impurity in soda ash prepared from trona. In the past, levels of soluble silicate in sodium carbonate mother liquors obtained from trona have been controlled or maintained by purging part of the mother liquor. This, of course, entailed loss of valuable soda values. It has also been proposed to reduce soluble silicate levels in aqueous solutions of sodium carbonate obtained by dissolving calcined trona in aqueous media by digesting the solutions at elevated temperature above about 160.degree. F. for time sufficient to substantially reduce the soluble silicates contained therein. Apparently, there is some interaction between the insoluble impurities in the process liquor and the soluble silicates, rendering insoluble or absorbing the soluble silicates to reduce their concentration in the liquor on digestion.
We have now made the unexpected discovery that water-insoluble matter contained in the fine particle size fraction of calcined trona, as compared to water-insoluble matter contained in the coarse particles size fraction, is much more effective in reducing soluble silicates in trona-derived carbonate liquors on digestion at elevated temperature.